Cluster ion mobility, structures, and binding energy: Application to the benzene cluster system

This dissertation describes the design, construction and operation of a mass-selected ion mobility mass spectrometer for the structural characterization of molecular cluster ions and the measurements of thermochemical properties. The ion mobility technique is applied to the study of benzene cluster ions. The mobility of benzene cluster ions (C₆H₆) _n+ n = 1–6 are reported. The experimentally determined structure of the benzene dimer cation, from mobility measurements in conjunction with density functional calculations, is shown to be a symmetrical stacked sandwich structure. Possible structures for the larger clusters, derived from the measured mobilities in conjunction with theoretical calculations obtained from Monte Carlo simulations are presented. Evidence for multiple isomers in the n = 3–6 clusters has been found, and growth patterns for the benzene cluster ion system are proposed.; The binding energy of the benzene dimer cation has been determined from the measurements of the temperature dependence of the monomer cation/dimer cation equilibrium constant. The equilibrium binding energy ΔH 0 and the entropy change ΔS0 of mass selected and isomer specific benzene dimer cations have been determined to be ΔH 0 = −17.7 kcal/mol, and −ΔS0 = 27.4 cal/mol K. Evidence for isomerization of the benzene radical cation in the electron impact ionization source is presented. The isomerization product is identified as fulvene ion, through a series of ionization energy bracketing experiments, and by utilizing the ion mobility techniques to determine the structure of the isomerized ion. The results reported in this dissertation open the door for systematic studies of the structure, binding energies and isomerization pathways of molecular cluster ions.